On the difference between YA and MG, the short answer is:
MG is for kids aged 8 or 9 to 12 years old, and YA is for 12 and up.
But the short answer is rarely the right one.
The first test of whether a book is MG or YA is the age of the protagonist. In general, the audience for the book should be about 2 years younger than the average reader. Therefore, the average reader of a MG book is between 10-11 years old, so the protagonist should be about 12 years old (about a grade higher up). There is a big difference between sixth graders and seventh graders, when those lovely hormones kick in.
But more than that, you have to look at the nature of the conflict:
- Internal conflict
- Local problems
- Threat can be big—i.e. the character needs to save the world—but motivation needs to be localized (character wants specifically to save family/friends)
- Less concern for worldly matters (i.e. national politics—will know who is president, but won't really care)
- Day to day concerns
- Limited mobility—usually restricted to home, school
- External conflict
- Larger problems
- Threat can be big, but character will consider the bigger picture in resolving the threat (i.e. will consider the consequences to the world, not just self, family, friends)
- Rising concern for worldly matters—for the first time knows and has an opinion about politics
- Concerned about the big picture on a long-term basis
- More freedom and mobility (can drive)